Aninfectionis the successful colonization of a host by a microorganism. Infections can lead to disease, which causes signs and symptoms resulting in a deviation from the normal structure or functioning of the host. Microorganisms that can cause disease are known as pathogens.
Thesignsof disease are objective and measurable, and can be directly observed by a clinician. Vital signs, which are used to measure the body’s basic functions, include body temperature (normally 37 °C [98.6 °F]), heart rate (normally 60–100 beats per minute), breathing rate (normally 12–18 breaths per minute), and blood pressure (normally between 90/60 and 120/80 mm Hg). Changes in any of the body’s vital signs may be indicative of disease. For example, having a fever (a body temperature significantly higher than 37 °C or 98.6 °F) is a sign of disease because it can be measured. In addition to changes in vital signs, other observable conditions may be considered signs of disease. For example, the presence of antibodies in a patient’s serum (the liquid portion of blood that lacks clotting factors) can be observed and measured through blood tests and, therefore, can be considered a sign. However, it is important to note that the presence of antibodies is not always a sign of an active disease. Antibodies can remain in the body long after an infection has resolved; also, they may develop in response to a pathogen that is in the body but not currently causing disease.
Michael, a 10-year-old boy in generally good health, went to a birthday party on Sunday with his family. He ate many different foods but was the only one in the family to eat the undercooked hot dogs served by the hosts. Monday morning, he woke up feeling achy and nauseous, and he was running a fever of 38 °C (100.4 °F). His parents, assuming Michael had caught the flu, made him stay home from school and limited his activities. But after 4 days, Michael began to experience severe headaches, and his fever spiked to 40 °C (104 °F). Growing worried, his parents finally decide to take Michael to a nearby clinic.
• What signs and symptoms is Michael experiencing?
• What do these signs and symptoms tell us about the stage of Michael’s disease? Jump to thenextClinical Focus box.
Unlike signs,symptomsof disease are subjective. Symptoms are felt or experienced by the patient, but they cannot be clinically confirmed or objectively measured. Examples of symptoms include nausea, loss of appetite, and pain. Such symptoms are important to consider when diagnosing disease, but they are subject to memory bias and are difficult to measure precisely. Some clinicians attempt to quantify symptoms by asking patients to assign a numerical value to their symptoms. For example, the Wong-Baker Faces pain-rating scale asks patients to rate their pain on a scale of 0–10. An alternative method of quantifying pain is measuring skin conductance fluctuations. These fluctuations reflect sweating due to skin sympathetic nerve activity resulting from the stressor of pain.
A specific group of signs and symptoms characteristic of a particular disease is called asyndrome. Many syndromes are named using a nomenclature based on signs and symptoms or the location of the disease.Table 15.1lists some of the prefixes and suffixes commonly used in naming syndromes.
Nomenclature of Symptoms
Affix Meaning Example
cyto- cell cytopenia: reduction in the number of blood cells
hepat- of the liver hepatitis: inflammation of the liver
-pathy disease neuropathy: a disease affecting nerves
-emia of the blood bacteremia: presence of bacteria in blood -itis inflammation colitis: inflammation of the colon
-lysis destruction hemolysis: destruction of red blood cells
-oma tumor lymphoma: cancer of the lymphatic system
-osis diseased or abnormal condition leukocytosis: abnormally high number of white blood cells -derma of the skin keratoderma: a thickening of the skin
Clinicians must rely on signs and on asking questions about symptoms, medical history, and the patient’s recent activities to identify a particular disease and the potential causative agent. Diagnosis is complicated by the fact that different microorganisms can cause similar signs and symptoms in a patient. For example, an individual presenting with symptoms of diarrhea may have been infected by one of a wide variety of pathogenic microorganisms. Bacterial pathogens associated with diarrheal disease includeVibrio cholerae,Listeria monocytogenes,Campylobacter jejuni, and enteropathogenicEscherichia coli(EPEC). Viral pathogens associated with diarrheal disease include norovirus and rotavirus. Parasitic pathogens associated with diarrhea include Giardia lambliaandCryptosporidium parvum. Likewise, fever is indicative of many types of infection, from the common cold to the deadly Ebola hemorrhagic fever.
Finally, some diseases may beasymptomaticorsubclinical, meaning they do not present any noticeable signs or symptoms. For example, most individual infected with herpes simplex virus remain asymptomatic and are unaware that they have been infected.
• Explain the difference between signs and symptoms.
1. F. Savino et al. “Pain Assessment in Children Undergoing Venipuncture: The Wong–Baker Faces Scale Versus Skin Conductance Fluctuations.”PeerJ1 (2013):e37; https://peerj.com/articles/37/